Automatic gain control (AGC) circuits are widely used in receivers for controlling the gain applied to a signal to be processed. In wireless communications, including mobile phones, communication signals are received via a receiver and then amplified for use in signal processing. Because the received signals often vary in signal strength, variable gain must be provided to allow the signal processor to recover the information within the signal. For example, in a code division multiple access (CDMA) system, the signal of interest has a large bandwidth, and it is necessary to limit the total power of the received signal for proper signal processing. Further, because the digital modulation technique of the CDMA system utilizes linear signal processing, the AGC circuits must prevent amplitude distortion of the signal.
A typical AGC circuit includes a variable gain amplifier (VGA) in a loop structure, a signal level detector, a control generator driven by the signal level detector and a stabilizing capacitor. In a CDMA system, the control of the VGA is set by the AGC loop, or is manually adjusted based upon externally received information. Hence, the performance of the control generator is key to the AGC loop in a CDMA system.
A commonly used means within wireless communications technologies for controlling the gain is the exponential (dB-linear) amplifier, in which the output of the amplifier is proportional to the exponential of the input voltage. As illustrated in FIG. 1, control current I.sub.X from current source 1 flows entirely through feedback transistor Q.sub.1. The fundamental relationship for the collector current of Q.sub.1 is: ##EQU1## where V.sub.T =KT/q and V.sub.O is the output voltage from operational amplifier 2. Using a similar expression for transistor Q.sub.2, ##EQU2## Comparing the two currents, I.sub.X and I.sub.O yields: ##EQU3## which simplifies to: ##EQU4##
As can be seen from Equation 4, the output current is affected by the temperature of the transistor Q.sub.1. It then becomes necessary to adjust the input voltage V to compensate for temperature changes in the thermal voltage, V.sub.T. Any error adjusting the input voltage V is magnified by the exponential function and leads to larger gain errors associated with the variable gain amplifier.